I think what they may be referring to is that people with certain types of lisp cannot pronounce an "s" sound in words, and instead pronounce the "s" as "th". If the "th" sound is exaggerated by drawing it out (thhhhh) it sounds somewhat like a hiss. What in the world this has to do with being gay is beyond me, but I have seen gay friends camping it up and part of the camp is drawing out the "s" sound in words, which creates a hissing sound.

Well, that's three people (including myself) who have never heard of the "gay hiss" thing.

Thank you Daniel the Apostate, Grundgetta, and Charles H Downs. I'm convinced my friend was confusing the hiss and the lisp. He couldn't elaborate further. He simply thought there was some stereotype involving hissing. Thank you all, once again. I just needed a couple more people backing me on this.